Pulp digester vent system

ABSTRACT

A system for controlling the venting of gases and vapors from a batch-type pulp digestion vessel includes a selectivelypositionable vent relief valve, a pressure sensor in the digestion vessel which generates signals representative of the pressures therein, and an electrical-pneumatic transducer associated with the relief valve for adjusting said relief valve proportionately and in response to the pressure signals, said relief valve being disposed for opening to a maximum when the signal amplitudes are a minimum and for continuous throttling toward a pre-selected restricted position as the amplitudes of the signals increase to a value corresponding to a pre-selected pressure in the digestion vessel.

O Umted States Patent [1 1 [H] 3,928,123

Marks [451 Dec. 23, 1975 PULP DIGESTER VENT SYSTEM 75 lnvcmon R be E Marks, Be" Fl Primary Examiner-William R. Cline l 0 a Attorney, Agent, or Firm-Robert E. Krebs; Thomas S. [73] Asstgnee: Envirotech Corporation, Menlo MacDonald Park, Calif.

[22] Filed: June 17, 1974 [57] ABSTRACT [21] App]. No.: 480,193 A system for controlling the venting of gases and vapets from a batch-type pulp digestion vessel includes a Application Data selectively-positionable vent relief valve, a pressure [63] Contmuatlon-in-part of Sen No. 294,42l, Oct. 2, sensor i h di i vesse| hi h generates i l I972, now Defensive Publication No. 3,8l7,824. representative of the pressures therein, and an electri, cal-pneumatic transducer associated with the relief 162/238, 137/87, 137/587 valve for adjusting said .relief valve proportionately [58] M I f g and in response to the pressure signals, said relief 3 valve being disposed for opening to a maximum when the signal amplitudes are a minimum and for continuous throttling toward a preselected restricted position [56] Referenms Cmd as the amplitudes of the signals increase to a value UNrrED STATES PATENTS corresponding to a pre-selected pressure in the diges- 2,3l0,4l5 2/1943 Frymoyer l62/6l (i011 vessel. 2,887,427 5/l959 Bennett 162/6] 2,896,709 7/1959 Ehrisman et a1. r. 162/238 13 Claim 6 Wing WWW MESS/R1250 A ll? SUPPL Y US. Patent Dec. 23, 1975 Sheet 2 of4 3,928,123

US. Patent Dec. 23, 1975 Sheet 3 of4 3,928,123

PULP DIGESTER VENT SYSTEM This application is a continuation-in-part of United States Patent application Ser. No. 294,42l filed Oct. 2, I972 which has matured into US. Pat. No. 3,817,824.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to an electrical-pneumatic system for automatically and continuously controlling wood chip digesters and, more particularly, to such a system for controlling the venting of vapors and gases from batch-type Kraft pulp digesters.

2. State of the Art In a conventional batch-type Kraft pulp digestion process, a mixture of wood chips and aqueous liquors is heated in a sealed pressure vessel for several hours under pressures ranging up to 140 psig. During cooking, the batch releases various gases and vapors, the latter which are referred herein as condensables. The gases principally are air and hydrogen sulfide; the condensable vapors generally include steam and on ganic turpene vapors. In some systems turpentine is recovered from the vapors by subsequent condensation and separation.

In such prior systems, a batch is initially heated with only a very restricted safety vent from the digester, thereby enabling immediate but very limited discharge of the cooking gases and vapors. By the addition of heat, the digester pressure is gradually increased to about 50-60 psig. At that pressure, the customary prior practice is to manually open a vent valve to preselected position to emit the bulk of the terpene vapors and, thereafter, to manually close the vent valve back to a single restricted position in order to save process steam. In other words, the valves were not continuously or gradually adjusted according to prior practice.

BRIEF SUMMARY OF THE INVENTION My co-pending US. patent application, Ser. No. 294,42! teaches an improved process for venting batch digester s. In view of my invention, the prior method of discharging digester gases and vapors is wasteful, especially with respect to thermal energy and turpene recovery. Particularly since the prior practice allows the non-condensable gases in the digest to mix with, or dilute, the organic vapors before venting, subsequent recovery of the organics by condensation was relatively inefficient. The steps in the process taught by my copending application generally comprise:

a. initiating heating of a batch at substantially atmospheric pressure in a closed digester;

b. at digester pressures of between two and ten psig,

opening a vent valve to its maximum (which releases mainly non-condensable gases);

c. then gradually and continuously reducing the vent opening while increasing digester pressure to a value ranging from about 80 to I20 psig;

d. maintaining thereafter substantially constant flow through the vent valve substantially constant digester pressure until digestion is complete. The present invention concerns ways and means for accomplishing and implementing the process set forth in my co-pending application.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention, including further advantages thereol, can be readily understood by reference to the following description and appended drawings, which are offered by way of example only and not in limitation of the invention whose scope is defined by the appended claims and equivalents. In the drawings:

FIGS. la and lb are a schematic diagram of a system according to the invention;

FIG. 2 is a graph illustrating operating conditions as will be later described in detail;

FIGS. 3 and 4 are schematic diagrams of a portion of the system of FIG. 1 in two different states; and

FIG. 5 is a graph illustrating other operating conditions as will be explained later in detail.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The batch-type pulp digestion system illustrated in FIG. 1 includes a pressure vessel 11 having a scalable top hatch 13 through which charge material is introduced, a bottom outlet 15 controlled by a heavy-duty on-off valve 16 for discharging digested pulp, and a main relief conduit 17 extending from the top of the vessel through which the cooking gases and vapors are vented. The vessel II can be heated either by introducing live steam into the batch or indirectly by using a heat exchanger. Typical cooking temperatures in the vessel range from 210 to 366 Fahrenheit and pressures range up to I50 psig. A conventional pressure sensor 19 and a conventional temperature sensor 21 are mounted in the pressure vessel and cooperatively function with the system to be described hereinafter to control venting and steam blow-back.

Preferably, the pressure is sensed above the operating liquid level or in the vapor head of the digester so that the readings do not reflect the hydrostatic pressure of the batch. The temperature sensor preferably measures the temperature of the batch itself and, accordingly, is located below the operating liquid level. In the drawings, a different arrangement of the sensors is shown for purposes of clarity. In some cases a plurality of spacedly-mounted temperature sensors can be utilized in order to obtain an average of the temperature within the batch.

FIG. 2 shows a typical saturation curve for steam, which curve is applicable to the liquids in a batch-type Kraft digestion process. For points lying on the curve (i.e., particular combinations of pressure and temperature), the liquids in the batch will just be at the boiling point. Boiling, of course, is a condition at which a liquid phase changes readily to a vapor phase. At points above the curve, the mixtures will not boil; to cause boiling, either the temperature in the digester must be increased and/or the pressure must be decreased. At points below the curve, the vapors are super-heated and boiling would still occur if the temperature were decreased or the pressure were increased.

According to the present invention, the batch in the digester is held under pressure-temperature conditions nearly exactly at the boiling point of the batch. In other words, the pressure within the digester is controlled such that at any given time, the digester pressure and temperature defines a point lying approximately on the saturation curve. Boiling is necessary so that the batch will cook properly and, also, so that steam will be evolved to serve as a carrier medium for the turpene or other recoverable vapors. On the other hand, excessive boiling of the batch (i.e., super-heating) is wasteful of thermal energy and does not significantly enhance digestion. Pulp cooking is optimally accomplished with the cooking liquors at temperatures above 21 2F (the boiling point at atmospheric pressure) and, therefore, cooking must be accomplished under somewhat pressurized conditions, as is conventional, in order that the cooking liquors reach the requisite elevated temperatures; according to the present invention, however, venting is implemented or profiled so that the digester pressures are not maintained at such magnitudes relative to temperature that boiling is precluded.

It should be appreciated that the curve in FIG. 2 is very nearly a straight line and that the ordinate scale is logarithmic. In other words, FIG. 2 illustrates that the saturation temperature is nearly linearly related, or almost directly proportional, to the logarithm of the saturation pressure. In still other words, to maintain the temperature-pressure conditions within the digester such that the saturation curve is followed, the difference between the logarithm of the pressure and a constant multiple of the temperature should be maintained relatively constant.

Referring again to FIG. 1, two pipes 27 and 31 branch from the main vent conduit 17 and conventional block valves 29 and 33 are interposed in the respective branches. The block valves are controlled so that usually one is fully open whenever the other is closed, and visa versa. The pipe 27 leads to a conventional separator (not shown) wherein the vented gases and vapors are separated and the organic vapors condensed to recover turpentine and other by-products. The branch pipe 31 leads to a steam supply which provides blow back" steam for cleaning the digester vent system. Typically, the blow back steam is at ISO to 200 psig.

The flow through the main conduit 17 is governed by a variable-blow relief valve 35. As will be explained further hereinafter, that valve is pneumatically controlled whereby elevated instrument air pressure opens the valve completely and diminishing air pressure throttles or restricts flow.

The electrical-pneumatic system in FIG. 1 automatically controls the aforementioned valves according to temperatures and pressures in the digester. At the beginning of the so-called pressure path" in that system, the pressure senor 19 is coupled to a conventional signal transducer 37 which converts the mechanical pressures readings to linearly related electrical currents which, preferably, range from ten to fifty milliamps. The current signals are carried, via lines 38 and 39, to a conventional logarithmic current amplifier 42 which converts the signals to a logarithmi output function, preferably a voltage. That is, the amp 'fier output is the logarithm of the amplitude of the input current signal. The logarithmic amplifier preferably includes a pair of calibration adjustments 45 and 46, the first of which is usually called a "zero" adjustment and the second a span adjustment, which are utilized to adjustably preset the amplifier output for given inputs.

Preferably, a conventional strip chart recorder 49 or the like is provided to display and record the output of the logarithmic amplifier. Since the electrical-driven pen assembly in the recorder has a high input impedances, it is preferable to connect the pen assembly across the logarithmic amplifier output lines 47 and 48 in parallel with a large resistor 51. The aforementioned calibration controls 45 and 46 on the logarithmic amplifier 42 are utilized to make adjustments to the logarithmic amplifier output so that the recorder 49 shows zero pressure when there is a milliamp input current to the logarithmic amplifier and reads full scale when there is a 50 milliamp input current.

In the illustrated system, a smaller resistor 52 is connected in series with the larger resistor 51 to provide a voltage divider so that a predetermined percentage of the voltage output of the logarithmic amplifier is developed across the smaller resistor. The voltage is them amplified by a conventional voltage-to-current amplifier 57. Preferably, the ratio" amplifier 57 utilizes negative current feedback and includes a variable potentiometer 59 so that the gain ratio can be selectively adjusted. In practice, the adjustment ranges from 0.4 to 1.8 times the input signal amplitude and the output current signals from the ratio" amplifier 57 vary from -l0 to -S0 milliamps. (The change of the sign is typical of such amplifiers.) Also, in practice, the ratio amplifier includes a conventional calibration adjustment 58 so that the maximum current output from the amplifier can be set at -l0 milliamps each time the gain ratio is changed. The current signals from the ratio amplifier 57 are carried, via lines 60 and 61 to a conventional electrical-pneumatic transducer 69 which converts the electrical signals into air pressures which are linearly related to the currents.

Preferably, a parallel combination of a normallyclosed electrical contact 63 and a resistor 64 are interposed in the output line 61 from the ratio amplifier. The contact 63 is governed by an automatic relay that will be described later herein. Because there is practically no voltage drop across the contact 63, the resistor 64 is shorted out of the circuit during normal operation.

Compressed air is supplied to the pneumatic system through a supply line 71. Communicating with that line is a conventional pressure switch 73 which is activated only if the supply pressure exceeds a pre-selected value, say about forty psig. Downstream from the pressure switch 73 are two branch lines 75 and 77, the first of which ultimately supplies air to the aforementioned transducer 69. A conventional pressure regulator 79 is interposed in the line 75 to maintain constant air pressure downstream, say at about 20 psig. Optionally, a conventional gauge 80 is provided to display the pressure.

As previously mentioned, the transducer 69 modulates the regulated air pressure carried by line 75 in linear relationship to the electrical signals which are received via the pressure sensor path. In practice, the modulated pressures range from about 3 to 15 psig.

The second branch conduit 77 from the supply line 71 also preferably includes an air pressure regulator 82 and an optional pressure indicator 83. The regulator 82 is set to maintain constant downstream pressure which, in practice, ranges from 45 to 50 psig. This higher pressure air is utilized in a sub-system as now be described for controlling the positions of the aforementioned block valves 29 and 33 to either vent the digestion vessel or to allow high pressure steam to flow thereinto through the relief valve 35.

Four conventional three-way valves 85-88, schematically illustrated in FIG. I, are connected to the high pressure line 77 by respective branch lines 89-92. The three-way valves selectively control the direction of air flow relative to two-way actuator assemblies 98 and 99 which are associated, respectively, with the aforementioned block valves 29 and 33. The positions of the thrceway vlaves are controlled by associated electrical solenoids 93-96, respectively. For example, the pres surized air supplied through the three-way valve 86 in FIG. I flows to the actuator assembly 98, but in FIG. 3 the associated solenoid 94 is de-energized so that the orientation of the three-way valve 86 is changed and the air flows through valve 86 to exhaust. Various conventional types of three-way valves can be utilized to accomplish such a result.

The acutator assemblies 98 nd 99 can be understood to include conventional, pneumatically-controlled pistons or diaphragms 98' and 99 which actuate the associated block valves 29 and 33. That is, the positions of the pistons determine whether the block valves are open or closed. In the illustrated system, the pistons are displaced by applying pressurized air against one of their faces and venting the opposite face to the atmosphere. The pneumatic lines associated with the pistons in those assemblies are designated 101-104; lines 101 and 103 must be pressurized to open the block valves, and lines 102 and 104 must be vented to close the block valves.

A variable-position actuator assembly 107 is coupled to the aforementioned variable vent valve 35. That assembly is conventional and includes a two-way piston 107 which is arranged to control the degree to which the vent valve is opened. More particularly, one end of the piston housing is connected by line 108 to receive relatively high pressure air from the regulator 82 so that the piston is urged to move in a direction which opens the valve. The other end of the piston assembly receives, via line 109, variable pressure instrument air from the transducer 69 which urges the piston to move in a direction which closes the valve. The actua tor assembly includes a biasing spring 110 so that the displacement of the piston is proportional to the applied pressure differential. Thus, the extend to which the relief valve is closed depends upon the pressure differential or balance between the constant highpressure supply and the modulated air pressure from the transducer 69.

The so-called temperature path will now be described. At the beginning of that path, the temperature sensor 21 is connected to a conventional signal converter 111 which linearly converts the thermal readings to instrument-compatable electrical current signals. in practice, the temperature sensor 21 and the converter Ill together typically comprise a conventional thermoelectric couple. The currents from the converter 111 preferably range from to 50 milliamps are are carried, via lines 113 and 114, to a voltage divider comprising a large resistor 117 and a smaller resistor H8 in series. The larger resistance "7 serves to match impedence with a second pen assembly that is included in the aforementioned strip chart recorder 49 to display and record the temperature transducer output and, hence the temperature in the digester.

As was previously mentioned, the temperature will be nearly linearly related to the logarithm of the pressure in the digester when the digester is operated to follow the saturation curve of FIG. 2. By the proper choice of resistances and scale in the recorder, the trace from the pen assembly which records the temperature can be very nearly superimposed upon the trace from the pen assembly which records the logarithm of the gauge pressure during an ideal digestion cycle. The deviation of the two traces during an actual cycle thusly shows the variance from the saturation conditions. It is very convenient and helpful to record such traces in practice.

A voltage comparing device 120, called the saturated pressure-temperature (SPT) comparator, is connected across the resistor 52 in the pressure path by lines I2] and 122 and, also, across the resistor 118 in the temperature path by lines 123 and 123'. The SPT comparator automatically compares the difference between the pressure path voltage and the temperature path voltage. The SPT voltage comparator is a conventional difference amplifier, and its output is coupled to the gate of an electronic trigger 124. The comparator is designed so that whenever the difference between the two voltages exceeds a certain preselected reference value, there is an indicative output from the comparator. Preferably, a potentiometer 125 is included in the comparator for selective adjustment of the reference voltage. In practice, if the difference voltage exceeds the reference, the indicative output is positive; but, if the difference voltage is less than the reference value, the comparator output is negative or zero. The potentiometer is called the saturated pressuretemperature" adjustment because it permits the reference voltage to be set at values which correspond to a differential between a multiple of the digester temperature and the logarithm of the digester pressure. In practice, the saturated pressure-temperature potentiometer is calibrated over a range which corresponds to 0.5 to 8 psig. The output from the comparator 120 is used in cooperation with various relays which will be described later herein to control the solenoids 93-96 associated with the previously-mentioned three-way valves 85-88.

Under certain circumstances, such as a severe blockage in the digester vent system, the pressure in the digester will increase beyond the saturation pressure and create a socalled false" pressure condition. Such a condition, in turn, causes the voltage comparator to yield a positive output, whose effects will be described later herein. False pressure can be understood to be a pressure-temperature condition in the digester which defines a point about the saturation curve in FIG. 2. At such a point, the batch will not boil because of the elevated pressure.

A second voltage comparator 128, called the lowpressure comparator, is also connected across the resistor 52 in the pressure path by lines 129 and 129'. That comparator compares the pressure path voltage with a preselected adjustable value called the low pressure" limit adjustment voltage determined by the settling of an associated potentiometer 130. Whenever the voltage drop across resistor 52 exceeds the reference voltage, there is a positive indicative output from this lowpressure comparator [28. In practice, the reference voltage is adjustable such that the comparator has an indicative output whenever the digester pressure exceeds a value in the range of 2 to ID psig', therefore, there is a positive output from the low pressure comparator during most of a normal digestion cycle and, only when the digester pressure is relatively low, will there not be a positive output from the comparator. Although the comparator 128 is illustrated as receiving the pressure-representative output signals from the logarithmic amplifier, the comparator could instead be connected to directly receive the output signals of the transducer 37. The output of the comparator 128 is coupled to the gate of a triggering element I31 whose use will be described later herein.

Primary power (generally l 15 volt, 60 cycle, single phase, alternating current) is applied to the electrical system at two terminals 132 and I33. From the first terminal 132, a line 135 connects directly to the first poles of each of the solenoids 93-96. A line 136 leads from the first terminal to the second terminal through a parallel arrangement of the two aforementioned electronic triggering devices 124 and 131. The practice, the triggering devices-are conventional components such as triacs, silicon-controlled rectifiers or the like which are typically utilized to control relatively large currents. Although the triggering elements are usually housed within the respective comparators 120 and 128, they are illustrated as being outside the comparators for purposes of clarity. The triggering elements do not conduct current unless they are gated open by electrical signals supplied to their gate terminals by the associated comparators.

A third line 137 from the first primary power terminal leads to the first poles of automatic relays and timers which will be further described later herein.

From the second primary power terminal 133, there are three parallel lines 142, 143, and 145. In the first line 142, there is contract 151 which is controlled by the aforementioned pressure switch 73 in the compressed air supply line 71; in the drawings, dashed lines indicate an association between a switch, or relay, and a contact. The contact 151 is closed only if the supply pressure exceeds a pre-selected minimum value determined by the pressure switch 73. A second contact 153 in the line 142 is controlled by a manual switch which may be regarded as the main on-off switch for the electrical system. In practice, the switch is called the auto switch because the system cannot operate automatically unless it is thrown.

A third contact 156 in the line 142 is actuated by the output from the low-pressure comparator 128. More particularly, the output from the comparator 128 is applied to the gate of the previously-mentioned triggering device 131. With the triggering device gated open, primary power flows through the line 136. lnterposed in that line is a conventional automatic relay 159 which, when power flows through it, reacts automatically to close the contact 156 in the line 142. As was previously mentioned, there is a positive output from the low-pressure comparator 128 only if the digester pressure exceeds a certain minimum value determined by the adjustable potentiometer 130. Therefore, the triggering device 131 conducts current and the contact 156 is closed only when the digester pressure exceeds the pre-selected minimum. This feature is important for two reasons. First, the setting of the low pressure comparator determines the minimum pressure at which the digester is vented. (In practice, a mechanical stop is provided so that the vent relief valve 35 is never completely closed but, nevertheless, there is no substantial venting at pressures below the pre-selected minimum.) Also, the low-pressure comparator is a safety feature insofar as there can be no steam blow-backs when the digester pressure is below the minimum as when the digester is open for loading or when discharging cooked pulp. The control of the blow-back system by the low-pressure comparator will be explained fully hereinafter.

A fourth contact 163 in the line 142 is ultimately controlled according to the condition of the aforementioned pulp discharge valve 16 which is governed by a conventional electrical actuator or positioned 165. During digestion, the pulp discharge valve 16 is closed and the actuator 165 is designed so that an associated electrical contact 169 is also closed, which contact 169 is interposed in the second line 143 between the primary power terminals 132 and 133. Also interposed in the line 143 is conventional automatic relay called the blow valve" relay because it is governed ultimately by the pulp discharge valve from which the pulp blows by internal pressure of the vessel 11 during discharge. When current flows through the line 143, the relay 175 automatically reacts to close the fourth contact 163 in line 141. Therefore, during normal digestion, the contacts 163 and 169 are always closed.

lnterposed finally in the line 141 is another automatic relay 179. When current flows through that relay, it reacts automatically to close an associated contact 183 which is interposed in the aforementioned line 145 which extends from the second primary power terminal 133. Several lines 186-189 connect to a node 190 on the other side of the contact 183.

A normally-closed contact 193 is interposed in the first line 186 from the node 190 and is actuated by an automatic relay in the timing circuit as will be described later therein. With the contact 193 normally closed, primary current flows directly through line 186 to energize the solenoids 94 and 96 which are associated with the three-way valves 86 and 88, respectively. ln other words, primary current flows through the solenoids 94 and 96 whenever the contacts 183 and 193 are closed in series.

The second line 187 from the node 190 leads directly to the solenoid 95 associated with the three-way valves 87. The line 187 is uninterrupted by further relays or the like and, accordingly, the solenoid is are immediately energized whenever the contact 183 is closed.

FIG. 1 shows the positions of the three-way valves 85-88 when the three solenoids 94-96 are energized, as indicated by the cross-hatching in the drawing. This is the normal or automatic" mode of the system, which is to say that digestion is occuring the pressure vessel, the digestion vapors are being vented and there is no blowback of steam. 1n the automatic mode, the steam blow-back valve 33 is fully closed, the separator block valve 29 is fully open, and the relief valve 35 is open proportionally to the air pressure determined by the transducer 69. More particularly, the action of the solenoid 96 positions the three-way valve 88 so that the valve closing line 104 exhausts through the three-way valve 88 and thence through the three-way valve 85, the latter being associated with the de-energized solenoid 93. The valve-opening line 101 for the steam block valve 33 also exhausts through the three-way valve 85. At the same time, pressurized air is directed through three-way valve 87 to the valve-closing side of the steam block valve actuator assembly 99 and through the three-way valve 86 to the valve-opening side of the separator block valve actuator assembly 99. The air flow directions are indicated by arrows.

Referring again to the electrical circuitry, the aforementioned conductor 137 leads from the first primary power terminal 132 to the first poles ofa set of conventional automatic timers 197-199 and, also, to an automatic multiple-contact relay 201 which is called the blowback" relay. All of the various switches and relays in the timing circuitry are shown in FIG. 1 in an orientation before a blowback sequence is initiated.

Prior to a blowback, current flows from the second primary power termianl 133 across the contact 183, thence through the conductor 189, across the arm of a switch 205 and through a conductor 203 to the pole of the first automatic timer 197 whose other pole, as was previously mentioned, is connected directly to the first primary ower terminal 132. The flow of current through the timer I97 initiates its timing action, which is preferably adjustably set for about minutes. The timer I97 is associated with a two-position switch 209 whose arm is normally in an opencircuit position. Upon elapse of the pro-selected time, the timer I97 moves the arm of the switch 209, thereby allowing current to flow to a second timer I98 via a conductor 21], so that the timer I98 is placed in a completed circuit. The timing cycle for timer 198 preferably is adjustable from O to 30 seconds.

With switch 209 in its second position, current also passes across the arm of a third switch 212 and thence to the aforementioned automatic blow-back relay 201 via a conductor 213. The blow-back relay is then ener gized since its other pole is connected to the first primary line terminal 132 via line I37. The state of the system following energization of the relay 201 is shown in FIG. 3.

When current flow energizes the blow-back relay 201, there are three simultaneous effects. First, the relay opens the associated normally-closed contact 63 that was described earlier in conjunction with current output of the ratio amplifier 57. The effect of opening the contact 63 is to force the ratio amplifier output current to flow through the aforementioned resistor 64 and, consequently, the current supplied to the electricalpneumatic transducer 69 will be substantially reduced which, in turn, causes the transducer to react to fully open the relief valve 35. In other words, the transducer 69 reacts to the opening of contact 63 by applying full pressure instrument air to the variable-positon actuator 107 to fully open the relief valve 35.

The second effect of actuation of the automatic blow-back relay 20I is the opening of the normallyclosed contact I93 that controls the flow of current to the solenoids 94 and 96. Opening of the contract I93 de-energizes these solenoids which, in turn, changes the orientations of the three-way valves 86 and 88 to the position in FIG. 3. In that orientation, the separator block valve 29 is closed due, first, to the passage of pressurized air through the three-way valve 88 to the closing side of the actuator assembly 98 and, secondly, due to the simultaneous exhausting of air from the actuator assembly 98 through line I03 and thence through the three-way valve 86.

The third simultaneous effect to energizing automatic blow-back relay 201 is to close a normally-open contact 2I6 in the line I88. Closure of that contact permits current to flow to energize the solenoid 93 which ultimately causes the stem block valve 33 to open. More particularly, the energized solenoid 93 actuates the three-way valve 85 to a position where exhaust is prevented and the pressurized supply air is passed directly to the "valve-opening" side of the steam block valve actuator assembly 99 via line IOI. Simultaneously, air is exhausted from the other end of the actuator assembly 99 through line 102 and thence through the inter-connected three-way valves 87 and 86. Such action opens the conduit 31 to permit pressurized steam to blow back into the digestion vessel 11. The duration of the blow-back is determined by the timing cycle of the timer I98 as will now be explained.

The timer 198 is operatively associated with the switch 2l2 so that the switch position is changed when the period set by that timer elapses. The effect of throwing switch 2I2 is to de-energize the automatic blow-back relay 201, which terminates the blow-back sequence. More particularly, the effect of de-energizing automatic relay 201 is to reverse the three previ ously mentioned occurances; namely the steam block valve 33 is again closed, the separator block valve 29 is again opened, and the contact 63 in the ratio amplifier output line 61 is again closed. The effect of the latter changes is to alleviate the inhibiting effect of the resistor 64 on the ratio amplifier output current so that the variable relief valve 35 is again positioned according to the digester pressure.

Another effect of the expiration of the cycle of the timer I98 is the placement of an automatic timer I99 in a completed circuit, which initiates its timing cycle. In practice, that latter timer cycle is adjustable up to 5 minutes, and is preferably set at three minutes. Timer 199 is called the disable" timer because block backs cannot occur until timer I99 completes its timing cycle. The disable timer 199 is associated with the arm of the aforementioned switch 205 and, as soon as the disable timer is energized to begin its timing cycle, the switch 205 is thrown to an open circuit position, which de-energizes the automatic blow-back relay 201 and, thereby prevents steam blow-backs. Also, the action de-energizes the timers 197 and 198 and causes the switches 209 and 212 to return to the position as shown in FIG. 1. After the disable timer 199 completes its timing cycle, the arm of associated switch arm 205 is moved to again bridge conductors 203 and 189. Timer I99 is also utilized to prevent continuous steam blow backs under certain conditions as will be explained hereinafter.

According to the system shown in FIG. I,steam blow backs are also initiated by operation of the saturated temperture-pressure comparator 120. As was mentioned previously, there is an indicative output from the comparator I23 whenever the digester pressure rises substantially above the saturation pressure and such a condition is called false pressure herein. Usually false pressure is caused by blockage of the relief system. In typical installations, a coarse screen is provided where the relief line I7 joins the top section of the digestion vessel. Ordinarily such a screen has about 3/16 inch apertures which may become blocked by the fibrous pulp or other materials so that ordinary venting is impaired. Steam blow back are provided to clean such a screen or other restrictions in the relief system. Otherwise, the pressures and temperatures within the digestion vessel II would build to potentially dangerous values.

The triggering device I24, discussed previously, allows current to flow through line I36 in response to positive outputs from the comparator I20. lnterposed in line I36 is an automatic relay 219 which is energized by the flow of current and which is associated with a normally open contact 221 interposed in a line 223 which interconnects lines 203 and 211 in the timing circuit. When relay 219 is energized, the contact 221 is closed and current can flow to energize the blow-back duration timer 198 via line 211 and, also, the automatic blow-back relay 201. In other words, the blow-back duration timer can be energized by the comparator I20 even if the interval timer I97 has not completed its timing cycle. The blow-back relay 20I, in turn, initiates a blow-back sequence whose duration is determined by the timer I98. Optionally, a manual switch 225 can be arranged in parallel with the contact 221 so that blow backs can be initiated manually.

It should be appreciated that under conditions of extreme blockage in the relief system, the comparator 120 would yield a continuous output which would cause continuous blow-backs but for disable timer 199 in the circuitry. It should further be appreciated that were the relief valve 35 undersized for a given application that, then, the blow-back sequence would be frequently initiated by the comparator 120; accordingly, the proper profile for the relief valve 35 can be deter mined by observing the blow-back frequency.

The low-pressure control circuitry by which blowbacks are precluded when the digester pressures are below a selected minimum value will now be explained. Low pressure conditions can occur, for example, while loading the digester with wood chips or while discharging the digested pulps. During such times, the blowing of high-pressure steam would be hazardous to person nel. As was discussed previously, the low-pressure comparator 128 has no output under low pressure conditions and, therefore, the triggering device 131 is not gated open. Consequently, there is no current flow through line 136 and the automatic relay 159 is deenergized and the contact 156 is opened. Then automatic relay 179 is consequently de-energized and contact 183 is opened. One effect of opening contact 183 is to de-energize the entire timing circuit, thereby preventing blow backs. Another effect is to de-energize all of the solenoids 93-96 associated with the three-way valves 85-88. The orientation of the three-way valves under such circumstances is shown in FIG. 4. Such orientation, called digester hold, would also occur in the event of a power failure or a loss of air pressure. In the latter circumstances, the pressure switch 73 will be deactivated and will open the usually closed contact I51. The latter action has the same effect upon the system as opening contact 156 as was previously described.

WORKING EXAMPLE This example shows how' the invention has been practiced, but should not be construed as limiting it. A 4500 cubic foot batch digester was charged with wood chips and Kraft cooking liquors. The cooking cycle had essentially the relief cycle characteristics of digester pressure and vent gas flow rate shown in FIG. 5, except that the discontinuities for steam blow backs are not indicated in that figure. The vent system utilized was that described in connection with FIG. I. The relief valve 35 was a heavy-duty, nominally 2-inch high-pressure V-port ball control valve. At the beginning of the cooking cycle, pressure built up autogenously in the digestion vessel to about 5 psig, and then the vent relief valve 35 was fully opened. The modulated air pressure from transducer 69 gradually throttled the vent relief valve 35 so that it had approximately the following characeristics: at 4 minutes it was I00 percent open; at minutes, about 90 percent; at minutes, about 70 percent; at 30 minutes, about 50 percent; at 40 minutes, about 37 percent; and at 45 minutes, about 32 percent.

The pressure in the digester gradually ascended from atmospheric pressure initially to l 10 psig at 90 minutes and thereafter was kept substantially constant until the end of the cook. Profiling of the venting was begun after about 4 minutes. After a short surge rich in noncondensables, the relief flow quantity increased gradu ally, and reached a maximum of about 900 pounds per hour at about 67 minutes. Thereafter, relief flow was maintained substantially constant. The cook was terminated after about two hours and the digested pulp was then blown. The operation showed a large process steam savings over prior practice as described hereinbefore Also, substantially turpentine began to flow in the vent relief cycle at 15 25 psig digester pressure (rather than the higher 5055 psig experienced with prior practice). Additionally, roughly 7% percent more turpentine was recovered.

I claim:

1. A system for controlling the venting of gases and vapors from a batch-type pulp digestion vessel comprising:

a. a selectively positionable relief valve connected in communication with the digestion vessel for venting vapors and gases therefrom;

b. means for continuously measuring the pressure in the digestion vessel;

c. means coupled to said pressure measuring means for generating first signals proportional in amplitude to the measured pressures in the digestion vessel;

d. means coupled to said first signal gene rating means for generating signals proportional in amplitude to the logarithms of said first signals; and

e. positioning means associated with said relief valve and connected to receive the logarithmic signals from said logarithmic signal generating means for adjusting said relief valve proportionately and in response to the logarithmic signals, said relief valve being disposed for opening to a maximum when the amplitudes of the logarithmic signals are a minimum and for continuous throttling toward a pre-selected restricted position as the amplitude of the logarithmic signals increase to a value corresponding to a pre-selected pressure in the digestion vessel.

2. A system according to claim 1 further including:

a. means for continuously measuring temperature in the digestion vessel;

b. means coupled to said temperature measuring means for generating signals proportional in magnitude to the measured temperatures in the digestion vessel;

c. first comparator means for determining the difference in magnitude between the pre-selected mulitple of the logarithmic signals and the temperature signals and for comparing the difference with a selectively adjustable reference value, said first comparator means yielding an indicative output when said difference exceeds said reference;

d. block valve means associated with said relief valve which, in a first position, permit the discharge of the vapors and gases through said relief valve along a first passageway leading therefrom and which, in a second position, close said first passageway and open a second passageway adapted for communication through said relief valve with a source of pressurized steam for introduction into said digestion vessel; and

e. actuator means operatively associated with said block valve means and responsive to the indicative outputs of said first comparator means to actuate said block valve means from said first position to said second position.

3. A system according to claim 2 further including:

a second comparator means coupled to said pressure measuring means for comparing the magnitude of signals representative of the pressure within the digestion vessel with a selectively adjustable reference value, said second comparator means yielding an indicative output when said reference value is exceeded; and

b. low-pressure control means associated with said actuator means and responsive to the indicative outputs from said second comparator means for precluding said block valve means from assuming said second position and, thereby, precluding the introduction of steam into the digestion vessel if the pressure in the digestion vessel is less than a pre-selected minimum value represented by said reference value associated with said second compartor means.

4. A system according to claim I further including:

a block valve means associated with said relief valve which, in a first position, permit the discharge of vapors and gases from the digestion vessel through said relief valve along a first passageway leading therefrom and which, in a second position, close said first passageway and open a second passageway adapted for communication through said relief valve with a source of pressurized steam for introduction into said digestion vessel;

b. actuator means operatively associated with said block valve means to actuate the same between said first and said second position;

c. first automatic timing means associated with said block valve actuator means for actuation of said block valves from said first position to said second position after the expiration of a a first pre-selected timing interval.

5. A system according to claim 4 further including second automatic timing means associated with said first timing means and said block valve actuator means for actuation of said block valve means from said second position to said first position after the expiration of a second pre-selected timing interval following the assumption of said second position by said block valve means.

6. A system according to claim 5 further including:

a. means for continuously measuring temperature in the digestion vessel;

b. means coupled to said temperature measuring means for generating signals proportional in magnitude to the measured temperatures in the digestion vessel;

c. first comparator means for determining the difference in magnitude between a pre-selected multiple of the logarithmic signals and the temperature signals and for comparing the difference with a selectively adjustable reference value, said first comparator means yielding an indicative output when said difference exceeds said reference;

d. blow-back initiating means responsive to the indicative outputs of said first comparator means and associated with said block valve actuator means to over-ride said first timing means to actuate said block valve means from said first position to said second position in response to said indicative outputs of said first comparator means.

7. A system according to claim 6 further including:

a. second comparator means coupled to said pressure measuring means for comparing the magnitude of signals representative of the pressures within the digestion vessel with a selectively adjustable reference value, said second comparator means yielding 14 an indicative output when the reference value is exceeded; and

b. low-pressure control means associated with said actuator means and responsive to the indicative outputs from said second comparator means for over-riding both said first timing means and said blow-back initiating means for precluding said block valve means from assuming said second position and, thereby, precluding the introduction of steam into the digestion vessel if the pressure in the digestion vessel is less than a pre-selected minimum value represented by said reference value associated with said second comparator means.

8. A system according to claim 5 further including disabling means associated with said second timing means and said block valve actuator means for precluding said block valves from returning to said second position from said first position for a third pre-selected timing interval following the expiration of said second timing interval.

9. A system according to claim 4 further including means associated with said first automatic timing means and said relief valve positioning means to open said relief valve fully upon the expiration of said first timing interval.

10. A system according to claim 1 wherein said positioning means includes an electrical-pneumatic transducer means which is connected to receive the logarithmic electrical signals and to modulate air pressure supplied from a regulated pressure source in direct relationship to those signals, and transducer means being associated with said relief valve so that the modulated pressurized air controls the opening of said relief valve.

11. A system according to claim 10 wherein said actuator means comprises at least one solenoidactuated pneumatic valve whose solenoid is connected to receive said outputs from said first comparator means for activation in response thereto to selectively direct pressurized air to actuate said block valve means.

12. A system for controlling the venting of gases and vapors from a batch-type pulp digestion vessel comprisa. a selectively positionable relief valve connected in communication with the digestion vessel for venting vapors and gases therefrom;

b. means for continuously measuring the pressure in the digestion vessel;

c. means coupled to said pressure measuring means for generating first signals representative of the measured pressures in the digestion vessel;

d. positioning means associated with said relief valve and connected to receive said representative signals from said first signal generating means for adjusting the opening of said relief valve proportionately and in response to said first signals, said relief valve being disposed for opening to a maximum when the pressure within the digester is at a minimum, as reflected by said representative signals, and for continuously throttling said relief valve toward a pre-selected restricted position as the pressure within the digester increases, as reflected by the amplitude of the representative signals, to a value corresponding to a pre-selected pressure in the digestion vessel.

13. A system according to claim l2 further including:

a. means coupled to said first signal generating means for generating signals proportional in amplitude to the logarithm of said first signals;

b. means for continuously measuring temperature in the digestion vessel;

c. means coupled to said temperature measuring means for generating signals proportional in magnitude to the measured temperatures in the digestion vessel;

d. first comparator means for determining the differences in magnitude between a preselected multiple of the logarithmic signals and the temperature signals and for comparing the difference with a selectively adjustable reference value, said first comparator means yielding an indicative output when said difference exceeds said reference; and

e. blowback initiating means for initiating a steam blowback in said digester in response to said indicative outputs from said first comparator means.

14. A system according to claim 13 including a block valve means associated with said relief valve which, in a first position, discharges vapors and gases through said relief valve along a first passageway leading therefrom and which, in a second position, closes said first passageway and opens a second passageway adapted for communication through said relief valve with a source of pressurized steam for introduction into said digestion vessel; and where said blowback initiating means comprises an actuator means operatively associated with said block valve means and responsive to the indicative outputs of said first comparator means to actuate said block valve means from said first position to said second position.

15. A system according to claim 14 further including;

a. second comparator means coupled to said pressure measuring means for comparing the magnitude of the signals representative of the pressures within the digestion vessel with a selectively adjustable reference value, said second comparator means yielding an indicative output when said reference value is exceeded; and

b. low-pressure control means associated with said actuator means and responsive to the indicative outputs from said second comparator means for precluding said block valve means from assuming said second position and. thereby, precluding the introduction of steam into the digestion vessel if the pressure in the degestion vessel is less than a preselected minimum value represented digestion said reference value associated with said second comparator means.

16. A system according to claim 12 further including:

a. block valve means associated with said relief valve which, in a first position, discharges vapors and gases from the digestion vessel through said relief valve along a first passageway leading therefrom and which, in a second position, closes said first passageway and opens a second passageway adapted for communication through the relief valve with a source of pressurized steam for introduction into said digestion vessel;

b. actuator means operatively associated with said block valve means to actuate the same between said first and said second position; and

c. first automatic timing means associated with said block valve actuator means for actuation of said block valves from said first position to said second position after the expiration of a first preselected timing interval.

17. A system according to claim 16 further including second automatic timing means associated with said first timing means and said block valve actuator means for actuation of said block valve means from said second position to said first position after the expiration of a second preselected timing interval following the assumption of said second position by said block valve means.

18. A system according to claim 16 further including means associated with said first automatic timing means and said relief valve positioning means to open and relief valve fully upon the expiration of said first timing interval. 

1. A system for controlling the venting of gases and vapors from a batch-type pulp digestion vessel comprising: a. a selectively positionable relief valve connected in communication with the digestion vessel for venting vapors and gases therefrom; b. means for continuously measuring the pressure in the digestion vessel; c. means coupled to said pressure measuring means for generating first signals proportional in amplitude to the measured pressures in the digestion vessel; d. means coupled to said first signal generating means for generating signals proportional in amplitude to the logarithms of said first signals; and e. positioning means associated with said relief valve and connected to receive the logarithmic signals from said logarithmic signal generating means for adjusting said relief valve proportionately and in response to the logarithmic signals, said relief valve being disposed for opening to a maximum when the amplitudes of the logarithmic signals are a minimum and for continuous throttling toward a pre-selected restricted position as the amplitude of the logarithmic signals increase to a value corresponding to a pre-selected pressure in the digestion vessel.
 2. A system according to claim 1 further including: a. means for continuously measuring temperature in the digestion vessel; b. means coupled to said temperature measuring means for generating signals proportional in magnitude to the measured temperatures in the digestion vessel; c. first comparator means for determining the difference in magnitude between the pre-selected mulitple of the logarithmic signals and the temperature signals and for comparing the difference with a selectively adjustable reference value, said first comparator means yielding an indicative output when said difference exceeds said reference; d. block valve means associated with said relief valve which, in a first position, permit the discharge of the vapors and gases through said relief valve along a first passageway leading therefrom and which, in a second position, close said first passageway and open a second passageway adapted for communication through said relief valve with a source of pressurized steam for introduction into said digestion vessel; and e. actuator means operatively associated with said block valve means and responsive to the indicative outputs of said first comparator means to actuate said block valve means from said first position to said second position.
 3. A system according to claim 2 further including: a. second comparator means coupled to said pressure measuring means for comparing the magnitude of signals representative of the pressure within the digestion vessel with a selectively adjustable reference value, said second comparator means yielding an indicative output when said reference value is exceeded; and b. low-pressure control means aSsociated with said actuator means and responsive to the indicative outputs from said second comparator means for precluding said block valve means from assuming said second position and, thereby, precluding the introduction of steam into the digestion vessel if the pressure in the digestion vessel is less than a pre-selected minimum value represented by said reference value associated with said second compartor means.
 4. A system according to claim 1 further including: a. block valve means associated with said relief valve which, in a first position, permit the discharge of vapors and gases from the digestion vessel through said relief valve along a first passageway leading therefrom and which, in a second position, close said first passageway and open a second passageway adapted for communication through said relief valve with a source of pressurized steam for introduction into said digestion vessel; b. actuator means operatively associated with said block valve means to actuate the same between said first and said second position; c. first automatic timing means associated with said block valve actuator means for actuation of said block valves from said first position to said second position after the expiration of a a first pre-selected timing interval.
 5. A system according to claim 4 further including second automatic timing means associated with said first timing means and said block valve actuator means for actuation of said block valve means from said second position to said first position after the expiration of a second pre-selected timing interval following the assumption of said second position by said block valve means.
 6. A system according to claim 5 further including: a. means for continuously measuring temperature in the digestion vessel; b. means coupled to said temperature measuring means for generating signals proportional in magnitude to the measured temperatures in the digestion vessel; c. first comparator means for determining the difference in magnitude between a pre-selected multiple of the logarithmic signals and the temperature signals and for comparing the difference with a selectively adjustable reference value, said first comparator means yielding an indicative output when said difference exceeds said reference; d. blow-back initiating means responsive to the indicative outputs of said first comparator means and associated with said block valve actuator means to over-ride said first timing means to actuate said block valve means from said first position to said second position in response to said indicative outputs of said first comparator means.
 7. A system according to claim 6 further including: a. second comparator means coupled to said pressure measuring means for comparing the magnitude of signals representative of the pressures within the digestion vessel with a selectively adjustable reference value, said second comparator means yielding an indicative output when the reference value is exceeded; and b. low-pressure control means associated with said actuator means and responsive to the indicative outputs from said second comparator means for over-riding both said first timing means and said blow-back initiating means for precluding said block valve means from assuming said second position and, thereby, precluding the introduction of steam into the digestion vessel if the pressure in the digestion vessel is less than a pre-selected minimum value represented by said reference value associated with said second comparator means.
 8. A system according to claim 5 further including disabling means associated with said second timing means and said block valve actuator means for precluding said block valves from returning to said second position from said first position for a third pre-selected timing interval following the expiration of said second timing interval.
 9. A system according to claim 4 further including means associated with said first automatic timing means and said reliEf valve positioning means to open said relief valve fully upon the expiration of said first timing interval.
 10. A system according to claim 1 wherein said positioning means includes an electrical-pneumatic transducer means which is connected to receive the logarithmic electrical signals and to modulate air pressure supplied from a regulated pressure source in direct relationship to those signals, and transducer means being associated with said relief valve so that the modulated pressurized air controls the opening of said relief valve.
 11. A system according to claim 10 wherein said actuator means comprises at least one solenoid-actuated pneumatic valve whose solenoid is connected to receive said outputs from said first comparator means for activation in response thereto to selectively direct pressurized air to actuate said block valve means.
 12. A SYSTEM FOR CONTROLLING THE VENTING OF GASES AND VAPORS FROM A BATCH-TYPE PULP DIGESTION VESSEL COMPRISING: A. A SELECTIVELY POSITIONABLE RELIEF VALVE CONNECTED IN COMMUNICATION WITH THE DIGESTION VESSEL FOR VENTING VAPORS AND GASES THEREFROM; B. MEANS FOR CONTINUOUSLY MEASURING THE PRESSURE IN THE DIGESTION VESSEL; C, MEANS COUPLED TO SAID PRESSURE MEASURING MEANS FOR GENERATING FIRST SIGNALS REPRESENTATIVE OF THE MEAUSRED PRESSURES IN THE DIGESTION VESSEL; D. POSITIONING MEANS ASSOCIATED WITH SAID RELIEF VALVE AND CONNECTED TO RECEIVE SAID REPRESENTATIVE SIGNALS FROM SAID FIRST SIGNAL GENERATING MEANS FOR ADJUSTING THE OPENING OF SAID RELIEF VALVE PROPORTIONATELY AND IN RESPONSE TO SAID FIRST SIGNALS, SAID RELIEF VALVE BEING DISPOSED FOR OPENING TO A MAXIMUM WHEN THE PRESSURE WITHIN THE DIGESTER IS AT A MINIMUM, AS REFLECTED BY SAID REPRESENTATIVE SIGNALS, AND FOR CONTINUOUSLY THROTTLING SAID RELIEF VALVE TOWARD A PRE-SELECTED RESTICTED POSITION AS THE PRESSURE WITHIN THE
 13. A system according to claim 12 further including: a. means coupled to said first signal generating means for generating signals proportional in amplitude to the logarithm of said first signals; b. means for continuously measuring temperature in the digestion vessel; c. means coupled to said temperature measuring means for generating signals proportional in magnitude to the measured temperatures in the digestion vessel; d. first comparator means for determining the differences in magnitude between a pre-selected multiple of the logarithmic signals and the temperature signals and for comparing the difference with a selectively adjustable reference value, said first comparator means yielding an indicative output when said difference exceeds said reference; and e. blowback initiating means for initiating a steam blowback in said digester in response to said indicative outputs from said first comparator means.
 14. A system according to claim 13 including a block valve means associated with said relief valve which, in a first position, discharges vapors and gases through said relief valve along a first passageway leading therefrom and which, in a second position, closes said first passageway and opens a second passageway adapted for communication through said relief valve with a source of pressurized steam for introduction into said digestion vessel; and where said blowback initiating means comprises an actuator means operatively associated with said block valve means and responsive to the indicative outputs of said first comparator means to actuate said block valve means from said first position to said second position.
 15. A system according to claim 14 further including; a. second comparator means coupled to said pressure measuring means for comparing the magnitude of the signals representative of the pressures within the digestion vessel with a selectively adjustable reference value, said second comparator means yielding an indicative Output when said reference value is exceeded; and b. low-pressure control means associated with said actuator means and responsive to the indicative outputs from said second comparator means for precluding said block valve means from assuming said second position and, thereby, precluding the introduction of steam into the digestion vessel if the pressure in the degestion vessel is less than a preselected minimum value represented digestion said reference value associated with said second comparator means.
 16. A system according to claim 12 further including: a. block valve means associated with said relief valve which, in a first position, discharges vapors and gases from the digestion vessel through said relief valve along a first passageway leading therefrom and which, in a second position, closes said first passageway and opens a second passageway adapted for communication through the relief valve with a source of pressurized steam for introduction into said digestion vessel; b. actuator means operatively associated with said block valve means to actuate the same between said first and said second position; and c. first automatic timing means associated with said block valve actuator means for actuation of said block valves from said first position to said second position after the expiration of a first preselected timing interval.
 17. A system according to claim 16 further including second automatic timing means associated with said first timing means and said block valve actuator means for actuation of said block valve means from said second position to said first position after the expiration of a second preselected timing interval following the assumption of said second position by said block valve means.
 18. A system according to claim 16 further including means associated with said first automatic timing means and said relief valve positioning means to open and relief valve fully upon the expiration of said first timing interval. 